Answer:
The radius of convergence is half of the length of the interval of convergence. If the radius of convergence is R then the interval of convergence will include the open interval: (a − R, a + R). To find the radius of convergence, R, you use the Ratio Test.
Answer:
No, he is not correct because the constant in his answer should have been 21 not 7.
Step-by-step explanation:

Therefore, the correct answer is the fourth; he should have put 21.
The proportion would be 180/x=60/x